Nonwoven webs typically are formed using a meltblowing process in which filaments are extruded from a series of small orifices while being attenuated into fibers using hot air or other attenuating fluid. The attenuated fibers are formed into a web on a remotely-located collector or other suitable surface. A spun bond process can also be used to form nonwoven webs. Spun bond nonwoven webs typically are formed by extruding molten filaments from a series of small orifices, exposing the filaments to a quench air treatment that solidifies at least the surface of the filaments, attenuating the at least partially solidified filaments into fibers using air or other fluid and collecting and optionally calendaring the fibers into a web. Spun bond nonwoven webs typically have less loft and greater stiffness than melt blown nonwoven webs, and the filaments for spun bond webs typically are extruded at lower temperatures than for melt blown webs.
There has been an ongoing effort to improve the uniformity of nonwoven webs. Web uniformity typically is evaluated based on factors such as basis weight, average fiber diameter, web thickness or porosity. Process variables such as material throughput, air flow rate, die to collector distance, and the like can be altered or controlled to improve nonwoven web uniformity. In addition, changes can be made in the design of the meltblowing or spun bond apparatus. References describing such measures include U.S. Pat. Nos. 4,889,476, 5,236,641, 5,248,247, 5,260,003, 5,582,907, 5,728,407, 5,891,482 and 5,993,943.
Despite many years of effort by various researchers, fabrication of commercially suitable nonwoven webs still requires careful adjustment of the process variables and apparatus parameters, and frequently requires that trial and error runs be performed in order to obtain satisfactory results. Fabrication of uniform wide nonwoven webs and of ultrafine fiber webs can be especially difficult.